Oxygen sensor deterioration detection method

ABSTRACT

A deterioration detection apparatus for an oxygen sensor is able to detect an abnormality of the oxygen sensor constantly with high precision, without being affected by the temperature characteristic of the element impedance. The apparatus applies a voltage V to the oxygen sensor, and calculates an element impedance real value Rsr=V/1 of a sensor element based on the applied voltage and the current I caused to flow by the voltage. The apparatus calculates an element temperature estimated value Tex of the oxygen sensor from a factor that affects the temperature of the oxygen sensor. The apparatus determines whether the oxygen sensor has an abnormality on the basis of whether the relationship between the element impedance real value Rsr and the element temperature estimated value Tex can be regarded as a relationship that agrees with a normal temperature characteristic.

INCORPORATION BY REFERENCE

This is a Division of application Ser. No. 10/706,929 filed Nov. 14,2003. The disclosure of the prior application is hereby incorporated byreference herein in its entirety.

The disclosure of Japanese Patent Application No. 2002-341327 filed onNov. 25, 2002, including the specification, drawings and abstractthereof, are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an oxygen sensor deterioration detectionapparatus and method, more particularly, to a deterioration detectionapparatus suitable for detecting deterioration of an oxygen sensor whoseelement impedance exhibits a temperature characteristic.

2. Description of the Related Art

A related-art apparatus that detects an abnormality of an oxygen sensoron the basis of an element impedance of the oxygen sensor is disclosedin Japanese Patent Application Laid-Open Publication No. 2000-198635. Inthis related-art detection apparatus, the oxygen sensor is disposed inan exhaust passageway of an internal combustion engine. The oxygensensor has a heater for heating a sensor element. In a normal serviceenvironment, the temperature of the sensor element is controlled atabout 700° C. via the heater and heat from exhaust gas.

In the above-described detection apparatus, a characteristic of theelement impedance of the sensor element is dependent on temperature.Therefore, if the heating of the sensor element is not appropriate, theelement impedance exhibits a value that is different from the valueexhibited when the heating is appropriate. Utilizing thischaracteristic, that is, on the basis of whether the element impedanceexhibits a normal value in a normal service environment, the detectionapparatus detects a broken wire related to the sensor, deterioration ofthe heater, a broken wire related to the heater, etc.

The detection apparatus assumes that the oxygen sensor has activated, atthe elapse of a predetermined time following startup of the internalcombustion engine. In that condition, the apparatus determines whetherthe oxygen sensor has an abnormality on the basis of the elementimpedance, on an assumption that the oxygen sensor has been heated toabout 700° C.

In reality, however, the temperature of the oxygen sensor may sometimesbe apart from the vicinity of 700° C. after the elapse of apredetermined time following startup of the internal combustion engine,even if the detection system is normal. In such a case, the value of theelement impedance greatly deviates from the value that is normallyexhibited when the temperature of the sensor element is at or around700° C., due to the temperature characteristic of the sensor element. Asa result, a false determination of an abnormality of the oxygen sensormay be made.

SUMMARY OF THE INVENTION

As an aspect of the invention, a first deterioration detection apparatusfor an oxygen sensor is provided. The detection apparatus includes afirst judgment value acquirer that calculates an element impedance realvalue from a value related to an electric power supplied to the oxygensensor, and that acquires the calculated value as a first judgmentvalue, a second judgment value acquirer which calculates an elementtemperature estimated value of the oxygen sensor from a factor thataffects a temperature of the oxygen sensor, and which acquires thecalculated value as a second judgment value, and an abnormalitydeterminer that determines whether the oxygen sensor has an abnormalitybased on the first judgment value and the second judgment value.

A deterioration detection method realized by the first deteriorationdetection apparatus is also provided. The deterioration detection methodincludes the steps of: calculating an element impedance real value froma value related to an electric power supplied to the oxygen sensor, andacquiring the calculated value as a first judgment value; calculating anelement temperature estimated value of the oxygen sensor from a factorthat affects a temperature of the oxygen sensor, and acquiring thecalculated value as a second judgment value; and determining whether theoxygen sensor has an abnormality based on the first judgment value andthe second judgment value.

As another aspect of the invention, a second deterioration detectionapparatus for an oxygen sensor is provided. This detection apparatusincludes a first judgment value acquirer that calculates an elementimpedance real value from a value related to an electric power suppliedto the oxygen sensor, and that acquires the calculated value as a firstjudgment value, a second judgment value acquirer which calculates anelement impedance estimated value from a factor that affects atemperature of the oxygen sensor, and which acquires the calculatedvalue as a second judgment value, and an abnormality determiner thatdetermines whether the oxygen sensor has an abnormality based on thefirst judgment value and the second judgment value.

A deterioration detection method realized by the second deteriorationdetection apparatus is also provided. The deterioration detection methodincludes the steps of: calculating an element impedance real value froma value related to an electric power supplied to the oxygen sensor, andacquiring the calculated value as a first judgment value; calculating anelement impedance estimated value from a factor that affects atemperature of the oxygen sensor, and acquiring the calculated value asa second judgment value; and determining whether the oxygen sensor hasan abnormality based on the first judgment value and the second judgmentvalue.

As still another aspect of the invention, a third deteriorationdetection apparatus for an oxygen sensor is provided. This detectionapparatus includes a first judgment value acquirer that calculates anelement temperature theoretical value from a value related to anelectric power supplied to the oxygen sensor, and that acquires thecalculated value as a first judgment value, a second judgment valueacquirer which calculates an element impedance estimated value from afactor that affects a temperature of the oxygen sensor, and whichacquires the calculated value as a second judgment value, and anabnormality determiner that determines whether the oxygen sensor has anabnormality based on the first judgment value and the second judgmentvalue.

A deterioration detection method realized by the third deteriorationdetection apparatus is also provided. This deterioration detectionmethod includes the steps of: calculating an element temperaturetheoretical value from a value related to an electric power supplied tothe oxygen sensor, and acquiring the calculated value as a firstjudgment value; calculating an element impedance estimated value from afactor that affects a temperature of the oxygen sensor, and acquiringthe calculated value as a second judgment value; and determining whetherthe oxygen sensor has an abnormality based on the first judgment valueand the second judgment value.

As a further aspect of the invention, a fourth deterioration detectionapparatus for an oxygen sensor is provided. This detection apparatusincludes a first judgment value acquirer that calculates an elementtemperature theoretical value from a value related to an electric powersupplied to the oxygen sensor, and that acquires the calculated value asa first judgment value, a second judgment value acquirer whichcalculates an element temperature estimated value from a factor thataffects a temperature of the oxygen sensor, and which acquires thecalculated value as a second judgment value, and an abnormalitydeterminer that determines whether the oxygen sensor has an abnormalitybased on the first judgment value and the second judgment value.

A deterioration detection method realized by the fourth deteriorationdetection apparatus is also provided. This deterioration detectionmethod includes the steps of: calculating an element temperaturetheoretical value from a value related to an electric power supplied tothe oxygen sensor, and acquiring the calculated value as a firstjudgment value; calculating an element temperature estimated value froma factor that affects a temperature of the oxygen sensor, and acquiringthe calculated value as a second judgment value; and determining whetherthe oxygen sensor has an abnormality based on the first judgment valueand the second judgment value.

According to the first to fourth deterioration detection apparatuses andmethods for an oxygen sensor, whether the oxygen sensor has anabnormality can be determined constantly with high precision, withoutbeing affected by the temperature characteristic of the elementimpedance.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned embodiments and other embodiments, objects, features,advantages, technical and industrial significance of the invention willbe better understood by reading the following detailed description ofexemplary embodiments of the invention, when considered in connectionwith the accompanying drawings, in which:

FIG. 1 is a block diagram for illustrating the construction according toan embodiment of the invention;

FIG. 2 is a diagram for illustrating a relationship between the elementimpedance and the element temperature of the sensor element;

FIG. 3 is a diagram for illustrating what judgment the apparatus of theembodiment makes with reference to the relationship between the elementtemperature estimated value Tex and the element impedance real valueRsr;

FIG. 4 is a flowchart illustrating a control routine executed by theapparatus according to the embodiment of the invention;

FIG. 5 is a flowchart illustrating a routine executed by the apparatusto estimate the element temperature of the oxygen sensor according tothe embodiment of the invention;

FIG. 6 is a flowchart illustrating a routine executed by the apparatusto detect a deterioration of the oxygen sensor according to theembodiment of the invention; and

FIG. 7 is a flowchart illustrating a routine executed by the apparatusto detect an intermediate short circuit of the oxygen sensor accordingto the embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description, the present invention will be described inmore detail in terms of exemplary embodiments.

Like elements and portions of the embodiments are represented by likereference numerals in the drawings, and will not be repeatedly describedbelow.

Embodiment

[Description of System Construction]

FIG. 1 is a block diagram for illustrating the construction ofEmbodiment 1 of the invention. Referring to FIG. 1, a system inaccordance with this embodiment has an oxygen sensor 10. In theembodiment, the oxygen sensor 10 is disposed in an exhaust passageway ofan internal combustion engine, and is used as a sensor for detecting theconcentration of oxygen in exhaust gas.

The oxygen sensor 10 has a sensor element 12, and a heater 14 forheating the sensor element 12. The sensor element 12 has characteristicsof generating an electromotive force corresponding to the oxygenconcentration in a detection-object gas, and changing an elementimpedance Rs in accordance with temperature. As indicated in FIG. 1, thesensor element 12 can be equivalently represented by an electromotiveforce component 16 and an impedance component 18.

The oxygen sensor 10 is connected to an ECU (electronic control unit)20. The ECU 20 is an on-vehicle computer formed by a CPU, a ROM, a RAM,electronic circuits, etc. In the embodiment, a heater control portion22, an element impedance detection portion 24, a fuel-cut detectionportion 26, an element temperature estimation portion 28 and anabnormality determination portion 30 are realized within the ECU 20 inhardware and software fashions.

The heater control portion 22 accomplishes a function of controlling theheater 14 of the oxygen sensor 10. The sensor element 12 of the oxygensensor 10 generates an electromotive force corresponding to the oxygenconcentration in a detection-object gas provided that the sensor element12 has been heated to a predetermined temperature. Therefore, the heatercontrol portion 22 controls the electrified and non-electrified statesof the heater 14 so that the activation temperature of the sensorelement 12 is maintained during operation of the oxygen sensor 10.

The element impedance detection portion 24 accomplishes a function ofdetecting the element impedance Rs of the sensor element 12. Morespecifically, the element impedance detection portion 24 supplies anappropriate electric power to the sensor element 12, and detects anelement impedance Rs from a value related to the supplied power. Sincethe sensor element 12 is an electromotive force type sensor, it isimpossible to extract a sensor output from the sensor element 12 duringsupply of power to the sensor element 12 (during application ofvoltage). Therefore, the element impedance detection portion 24 suppliespower to the sensor element 12 only at a predetermined timing at whichthe element impedance Rs is to be detected. Then, the element impedancedetection portion 24 detects the element impedance Rs in accordance witha relationship (V=Rs·I) between the applied voltage V across the sensorelement 12 and the current I through the sensor element 12. Hereinafter,the element impedance Rs detected by the element impedance detectionportion 24, that is, the element impedance Rs calculated from valuesrelated to the electric power supplied to the sensor element 12, will bereferred to as “element impedance real value Rsr”.

The element impedance detection portion 24 is provided with a firstjudgment value acquirer that calculates the element impedance real valueRsr, and that acquires the calculated value as a judgment value (firstjudgment value).

In the internal combustion engine, fuel-cut for stopping the injectionof fuel is executed, for example, if the engine rotation speed is highand an accelerator pedal is released. The fuel-cut detection portion 26accomplishes a function of detecting execution of the fuel-cut if theabove-described fuel-cut is executed in the internal combustion engine.A result of detection provided by the fuel-cut detection portion 26 issupplied to the element temperature estimation portion 28.

The element temperature estimation portion 28 is supplied withinformation regarding the execution of fuel-cut from the fuel-cutdetection portion 26 as stated above, and is also supplied withinformation regarding the electrification/non-electrification of theheater 14 from the heater control portion 22. The element temperatureestimation portion 28 is connected to an air flow meter 32, a vehiclespeed sensor 34, an intake air temperature sensor 36, and an atmosphericpressure sensor 38. Therefore, the element temperature estimationportion 28 is supplied with information regarding the amount of intakeair Ga, the vehicle speed SPD, the intake air temperature Ta, and theatmospheric pressure Pa from those sensors. All the aforementionedpieces of information supplied to the element temperature estimationportion 28 concern factors that affect the temperature of the oxygensensor 10 (sensor element 12). More specifically, the pieces ofinformation concern major factors that affect the supplied amount ofheat Ts for the oxygen sensor 10 (sensor element 12) or the amount ofradiant heat Tr from the oxygen sensor 10 (sensor element 12). On thebasis of the aforementioned information, the element temperatureestimation portion 28 accomplishes the function of estimating thetemperature of the sensor element 12. Hereinafter, the temperature ofthe sensor element 12 estimated by the element temperature estimationportion 28, that is, the element temperature calculated on the basis ofthe information regarding major factors that affect the temperature ofthe sensor element 12, will be referred to as “element temperatureestimated value Tex”.

The element temperature estimation portion 28 is provided with a secondjudgment value acquirer that calculates the element temperatureestimated value Tex and acquires the calculated value as a judgmentvalue (second judgment value).

The abnormality determination portion 30 determines whether the oxygensensor 10 has an abnormality on the basis of the element impedance realvalue Rsr detected by the element impedance detection portion 24, andthe element temperature estimated value Tex estimated by the elementtemperature estimation portion 28.

The abnormality determination portion 30 is provided with an abnormalitydeterminer that determines whether the oxygen sensor 10 has anabnormality on the basis of the first judgment value acquired by thefirst judgment value acquirer and the second judgment value acquired bythe second judgment value acquirer. The abnormality determiner isequipped with a heater electrification state detector that detects theelectrified state of the heater described below, a sensor elementabnormality identifier that, if it is determined that the oxygen sensorhas an abnormality in a situation where the heater is not electrified,identifies the abnormality as an abnormality of the sensor element, andan electrification stopper that stops electrification of the heater ifit is determined that the oxygen sensor has an abnormality in asituation where the heater is electrified. The abnormality determiner isfurther equipped with a first change amount detector that detects theamount of change in the first judgment value, and a second change amountdetector that detects the amount of change in the second judgment value.

FIG. 2 indicates a relationship between the element impedance Rs and theelement temperature of the sensor element 12, that is, a temperaturecharacteristic exhibited by the element impedance Rs of the sensorelement 12. As indicated in FIG. 2, the sensor element 12 has acharacteristic of decreasing the element impedance Rs exponentially withrespect to increases in the element temperature.

If the oxygen sensor 10 is normal, the correlation as indicated in FIG.2 should exist between the element impedance real value Rsr calculatedfrom electric power-related values and the element temperature estimatedvalue Tex estimated on the basis of a factor that affects thetemperature. Therefore, the abnormality determination portion 30determines whether the oxygen sensor 10 has an abnormality on the basisof whether a normal correlation as indicated in FIG. 2 is found betweenthe element impedance real value Rsr and the element temperatureestimated value Tex.

[DESCRIPTION OF SPECIFIC CRITERIA FOR ABNORMALITY DETERMINATION] FIG. 3is a diagram for illustrating what judgment the apparatus of theembodiment makes with reference to the relationship between the elementtemperature estimated value Tex and the element impedance real valueRsr. In FIG. 3, T1 and T2 on the horizontal axis indicate a lower-limitvalue and an upper-limit value, respectively, of a normal operationtemperature range of the oxygen sensor 10. More specifically, thetemperature T1 is a lower-limit temperature (e.g., 350° C.) fordetermining that the oxygen sensor 10 is an activated state. Thetemperature T2 is an upper-limit temperature (e.g., 900° C.) that theoxygen sensor 10 can reach in a normal service environment.

In FIG. 3, R1 on the vertical axis indicates an upper limit of theresistance value that will be reached only when there is a broken wirerelated to the sensor element 12 provided that the oxygen sensor 10 isin the activated state (provided that the temperature is at or aboveT1), that is, a value (e.g., 15 kΩ) that is sufficiently larger than theelement impedance Rs that the sensor element 12 normally exhibits at thetemperature T1. Furthermore, R2 on the vertical axis indicates a lowerlimit of the resistance value that will be exhibited only when thesensor element 12 has a short circuit provided that the oxygen sensor 10is used in a normal service environment (provided that the temperatureis at or below T2), that is, a value (e.g., 5 Ω) that is sufficientlysmaller than the element impedance Rs that the sensor element 12normally exhibits at the temperature T2.

With the above-described settings, it can be judged that the sensorelement 12 has a broken wire if the element impedance real value Rsr isgreater than R1 although the element temperature estimated value Tex ishigher than T1 (region (1)). If the element impedance real value Rsr isless than R2 although the element temperature estimated value Tex islower than T2 (region (2)), it can be judged that the sensor element 12has a short circuit. Therefore, in the embodiment, the ECU 20 judgesthat the sensor element 12 has a broken wire if the combination of theelement temperature estimated value Tex and the element impedance realvalue Rsr is within the region (1). If that combination is within theregion (2), the ECU 20 judges that the sensor element 12 has a shortcircuit.

In FIG. 3, the straight line denoted by reference numeral (3) indicatesa temperature characteristic that the element temperature and theelement impedance Rs normally exhibit. If the oxygen sensor 10 isnormal, the combination of the element temperature estimated value Texand the element impedance real value Rsr should be near the straightline (3) provided that the element temperature estimated value Tex iswithin the normal range (provided that T1<Tex<T2 holds). That is, if thecombination of the element temperature estimated value Tex and theelement impedance real value Rsr is far apart or remote from thestraight line (3) in a situation where T1<Tex<T2 holds, it can be judgedthat the oxygen sensor 10 is not normal.

In FIG. 3, the straight lines denoted by reference numerals (4) and (5)are sets of points of boundary for determining that the combination ofthe element temperature estimated value Tex and the element impedancereal value Rsr is far apart from the straight line (3). Therefore, inthe embodiment, the ECU 20 determines that the sensor element 12 hasdeteriorated, if the combination of the element temperature estimatedvalue Tex and the element impedance real value Rsr is in a region (6) ora region (7) shown in FIG. 3.

If the oxygen sensor 10 is normal, a correlation that agrees with thenormal temperature characteristic should be found between the elementtemperature estimated value Tex and the element impedance real valueRsr. That is, as long as the oxygen sensor 10 is normal, a change thatoccurs in one of the element temperature estimated value Tex and theelement impedance real value Rsr should be accompanied with anappropriate change in the other that corresponds to the aforementionedchange. Therefore, it can be determined that the oxygen sensor 10 has anabnormality if a normal correlation is not found between a change ATexin the element temperature estimated value Tex and a change ARsr in theelement impedance real value Rsr regardless of which region thecombination of the element temperature estimated value Tex and theelement impedance real value Rsr exists in (regardless of whether thecombination is in a region (8) between the straight lines (4) and (5)).In the embodiment, if a phenomenon as stated above is detected, the ECUdetermines that the sensor element 12 has a short circuit with anintermediate impedance (hereinafter, referred to as “intermediate shortcircuit”).

[DESCRIPTION OF SPECIFIC PROCESS EXECUTED FOR ABNORMALITY DETERMINATION]The contents of specific processes executed by the ECU 20 to determinewhether the oxygen sensor 10 has an abnormality in accordance with theaforementioned specific criteria will be described with reference toFIGS. 4 to 7. FIG. 4 is a flowchart illustrating a control routineexecuted by the ECU 20 to detect a broken wire and a short circuit ofthe sensor element 12. In the routine illustrated in FIG. 4, an elementtemperature estimated value Tex of the oxygen sensor 10 is firstcalculated (step 100).

FIG. 5 is a flowchart illustrating the content of an element temperatureestimating process executed in step 100. The element temperatureestimating process is accomplished by the second judgment valueacquirer.

Every time the step 100 is executed, the ECU 20 calculates an elementtemperature estimated value Tex following the routine illustrated inFIG. 5. In the routine illustrated in FIG. 5, the element temperatureestimated value Tex and the supplied amount of heat Ts calculated in theprevious execution cycle are substituted for a former elementtemperature estimated value Tex0 and a former supplied amount of heatTs0, respectively (step 108). The aforementioned supplied amount of heatTs is an amount of heat supplied to the sensor element 12. Details ofthe supplied amount of heat Ts and the calculation method the suppliedamount of heat Ts will be described later.

In the routine illustrated in FIG. 5, the amount of intake air Ga, thevehicle speed SPD, the intake air temperature Ta and the atmosphericpressure Pa are subsequently detected via the various sensors connectedto the ECU 20 (step 110).

Subsequently, information regarding the state of execution of thefuel-cut and the state of electrification of the heater 14 is detectedvia the fuel-cut portion detection portion 26 and the heater controlportion 22 (step 112).

Next, an element temperature convergence value Tga corresponding to thepresent situation is calculated on the basis of the amount of intake airGa, the state of execution of the fuel-cut and the state ofelectrification/non-electrification of the heater (step 114).

The element temperature convergence value Tga is greatly affected by theamount of flow and the temperature of exhaust gas. The convergence valueTga is also greatly affected by whether the heating by the heater 14 isbeing performed. The amount of flow of exhaust gas is, in principle,equal to the amount of intake air Ga. The temperature of exhaust gasgreatly varies depending on whether the fuel-cut is being executed.Therefore, in the embodiment, the element temperature convergence valueTga is calculated on the basis of the amount of intake air Ga, the stateof execution of the fuel-cut, and the state ofelectrification/non-electrification of the heater, as stated above. TheECU 20 stores a map that determines the element temperature convergencevalue Tga in relation to the aforementioned three factors. Specifically,in step 114, the element temperature convergence value Tga is determinedby specifying the three factors in the map.

In the routine illustrated in FIG. 5, an atmospheric pressure correctioncoefficient Kp is subsequently calculated on the basis of theatmospheric pressure Pa.

The temperature of the sensor element 12 more greatly changes if thepresent temperature is farther apart from the element temperatureconvergence value Tga. That is, the amount of heat Ts supplied to thesensor element 12 has a correlation with the difference between thepresent element temperature and the element temperature convergencevalue Tga. This correlation changes with the atmospheric pressure Pa,due to the effect of air density or the like. The atmospheric pressurecorrection coefficient Kp is a coefficient that is determined as a valueto be added to the element temperature convergence value Tga.

After the atmospheric pressure correction coefficient Kp is computed, anamount Ts of heat supplied to the sensor element 12 is computed as inthe following equation (step 118).Ts=Ts0+[{(Tga·Kp−Tex0)/Kb}−Ts0]/Ka   (1)

In equation (1), Ts0 and Tex0 are the supplied amount of heat Ts and theelement temperature estimated value Tex computed in the previousexecution cycle (see step 108). Furthermore, in equation (1), both Kband Ka are annealing constants. In the right-hand side of equation (1),the term (Tga·Kp−Tex0) means the amount of heat supplied to the sensorelement 12 in the present execution cycle. The term {(Tga·Kp−Tex0)/Kb}is an annealed value of the amount of heat (Tga·Kp−Tex0). Furthermore,the term [{(Tga·Kp−Tex0)/Kb}−Ts0] means the difference between thesupplied amount of heat Ts0 determined in the previous execution cycleand the annealed value of the supplied amount of heat calculated in thepresent execution cycle. According to equation (1), the supplied amountof heat Ts in the present execution cycle is determined by reflecting,in the former supplied amount of heat Ts0, the value obtained byannealing the difference in the supplied amount of heat via Ka (byadding the Ka-annealed value to the former supplied amount of heat Ts0).

According to the processing of step 118, the heat environment existingat the time of the present execution cycle is appropriately reflected ina base, that is, the supplied amount of heat Ts0 determined in theprevious execution cycle, so that the amount Ts of heat supplied to thesensor element 12 can be accurately calculated.

In the routine illustrated in FIG. 5, a radiant heat coefficient Kspdrelated to the vehicle speed SPD is calculated (step 120).

While the vehicle is running, the oxygen sensor 10 is cooled byvehicle-run ventilation. The radiant heat coefficient Kspd is acoefficient related to the amount of heat radiating from the oxygensensor 10 due to the cooling by the vehicle-run ventilation. The valueof the radiant heat coefficient Kspd increases with increasing vehiclespeed SPD. The ECU 20 stores a map that determines a relationshipbetween the vehicle speed SPD and the radiant heat coefficient Kspd. Instep 120, the radiant heat coefficient Kspd is determined in accordancewith the map.

After the radiant heat coefficient Kspd is determined, an amount ofradiant heat Tr from the oxygen sensor 10 is calculated as in thefollowing equation (step 122).Tr=Kspd·(Tex−Ta)   (2)

The amount of radiant heat Tr from the oxygen sensor 10 is greatlyaffected by the difference between the element temperature estimatedvalue Tex and the intake air temperature Ta, as well as the vehiclespeed SPD. According to equation (2), the amount of radiant heat Tr fromthe oxygen sensor 10 can be accurately calculated by taking the twofactors into account.

Due to the above-described series of steps, the amount of heat Trradiating from the oxygen sensor 10 and the amount Ts of heat suppliedto the oxygen sensor 10 in the present execution cycle can becalculated. The ECU 20 calculates the element temperature estimatedvalue Tex in the present execution cycle by substituting the amounts ofheat Ts, Tr and the former element temperature estimated value Tex0 inthe following equation (step 124).Tex=Tex0+(Ts−Tr)   (3)

With reference to the flowchart of FIG. 4 again, the control routinewill be described.

After the element temperature estimated value Tex is calculated in step100 in the routine illustrated in FIG. 4, an element impedance realvalue Rsr is calculated (step 130). A process of judging the elementimpedance real value Rsr in step 130 is accomplished by the firstjudgment value acquirer.

The element impedance real value Rsr is calculated on the basis of avalue related to the electric power supplied to the oxygen sensor 10.More specifically, for calculation of the element impedance real valueRsr, the ECU 20 first applies a predetermined voltage V to the oxygensensor 10. The ECU 20 then detects the current I that flows through thesensor element 12 due to application of the voltage V, and calculatesthe element impedance real value Rsr=V/I on the basis of the appliedvoltage V and the thereby-caused current I.

In the routine illustrated in FIG. 4, it is determined whether theelement impedance real value Rsr is smaller than a broken wire criterionRI (e.g., 15 Ω) (step 132).

If it is determined that Rsr<R1 does not hold, that is, if it isdetermined that the element impedance real value Rsr is greater than orequal to the broken wire criterion R1, it is subsequently determinedwhether the element temperature estimated value Tex is higher than anactivation criterion T1 (e.g., 350° C.) (step 134).

If it is determined that the element temperature estimated value Tex isnot higher than the activation criterion T1, it can be judged that thereis a possibility of the element impedance real value Rsr exceeding thebroken wire criterion R1 because the oxygen sensor 10 is not activated.In this case, the determination regarding the state of the oxygen sensor10 is suspended. After that, the present execution cycle immediatelyends.

Conversely, if it is determined in step 134 that the element temperatureestimated value Tex is higher than the activation criterion temperatureT1, it can be judged that a falsely great value of the element impedanceRs is presented despite the activation of the oxygen sensor 10. In thiscase, the ECU 20 determines that the sensor element 12 has a brokenwire, and ends the present execution cycle (step 136).

If it is determined in step 132 that the element impedance real valueRsr is less than the broken wire criterion R1, it is subsequentlydetermined whether the element impedance real value Rsr is greater thana short-circuit criterion R2 (step 138).

If it is determined that Rsr>R2 does not hold, that is, if it isdetermined that the element impedance real value Rsr is less than orequal to the short-circuit criterion R2, it is subsequently determinedwhether the element temperature estimated value Tex is lower than anormal upper-limit temperature T2 (e.g., 900° C.) (step 140).

If it is determined that the element temperature estimated value Tex isnot lower than the normal upper-limit temperature T2, it can be judgedthat there is a possibility of the element impedance real value Rsrbeing less than or equal to the short-circuit criterion R2 due to a hightemperature of the oxygen sensor 10 above the normal temperature. Inthis case, the determination regarding the state of the oxygen sensor 10is suspended. After that, a below-described process starting at step 144is executed.

Conversely, if it is determined in step 140 that the element temperatureestimated value Tex is lower than the normal upper-limit temperature T2,it can be judged that a falsely small value of the element impedance Rsof the oxygen sensor 10 is presented. In this case, the ECU 20determines that there is a short circuit regarding the sensor element12, and then ends the present execution cycle (step 142).

In the case where it is determined in step 138 that Rsr>R2 holds, and inthe case where it is determined in step 140 that Tex<T2 does not hold,the ECU 20 determines that the oxygen sensor 10 is in the activatedstate. Therefore, the ECU 20 executes a known air-fuel ratio feedbackcontrol on the basis of the output of the oxygen sensor 10 (step 144).

After that, the ECU 20 executes a deterioration detection process fordetecting deterioration of the oxygen sensor 10 (described below withreference to FIG. 6) and a short-circuit detection process for detectingan intermediate short circuit of the oxygen sensor 10 (described belowwith reference to FIG. 7), and ends the present execution cycle (step146).

It is to be noted that the process of steps 132 to 146 is accomplishedby the aforementioned abnormality determiner.

FIG. 6 shows a flowchart illustrating a deterioration detection processexecuted in step 146 by the ECU 20.

In the routine illustrated in FIG. 6, an element temperature theoreticalvalue Ti is first calculated on the basis of the element impedance realvalue Rsr (step 150).

The element temperature and the element impedance Rs of the oxygensensor 10 has a relationship (temperature characteristic) as indicatedin FIG. 2, as described above. The ECU 20 stores a map corresponding tothe relationship. In step 150, the ECU 20 calculates an elementtemperature theoretical value Ti corresponding to the element impedancereal value Rsr in accordance with the map.

Subsequently, it is determined whether the difference |Tex−Ti| betweenthe element temperature estimated value Tex and the element temperaturetheoretical value Ti is greater than a criterion T3 (e.g., 100° C.)(step 152).

The element temperature estimated value Tex is an element temperaturecalculated on the basis of a value that affects the temperature of theoxygen sensor 10. The element temperature theoretical value Ti is anelement temperature calculated as a value corresponding to the elementimpedance real value Rsr, that is, an element temperature calculated onthe basis of a value related to the electric power of the oxygen sensor10. The two physical quantities, that is, the element temperatureestimated value Tex and the element temperature theoretical value Ti,should coincide with each other if the oxygen sensor 10 exhibits anormal temperature characteristic. Therefore, if the difference betweenthe two |Tex−Ti| is small, it can be judged that the temperaturecharacteristic exhibited by the oxygen sensor 10 is normal. Conversely,if the difference |Tex−Ti| is large, it can be judged that thetemperature characteristic exhibited by the oxygen sensor 10 is notnormal.

The processing of step 152 is equivalent to a process of determiningwhether the combination of the element temperature estimated value Texand the element impedance real value Rsr is within the region (6) or (7)indicated in FIG. 3.

In the routine illustrated in FIG. 6, if it is determined in step 152that |Tex−Ti|>T3 does not hold, it is judged that the oxygen sensor 10is normal. After that, the routine illustrated in FIG. 6 is promptlyended. Conversely, if it is determined that |Tex−Ti|>T3 holds, it isjudged that the oxygen sensor 10 has deteriorated (step 154).

The condition examined in step 152 is fulfilled in a case where theheater 14 has an abnormality although the sensor element 12 is normal,as well as the case where the sensor element 12 has deteriorated. Thatis, in the embodiment, during the course of calculating the elementtemperature estimated value Tex, the supplied amount of heat Ts (moreprecisely, the element temperature convergence value Tga) differsdepending on whether the heater 14 is in an electrified state or anon-electrified state (see step 114). If in an environment where theheater 14 does not perform normal function, the element temperatureconvergence value Tga is calculated on assumption that the heater 14 iselectrified, a large error is superimposed on the element temperatureestimated value Tex, so that the condition in step 152 may be satisfied.Hence, if it is determined in step 152 that the oxygen sensor 10 hasdeteriorated, step 152 is followed by a process for specificallydetermining whether the deterioration of the oxygen sensor 10 isdeterioration of the sensor element 12 or deterioration of the heater14.

Specifically, subsequently to step 154, the ECU 20 determines whetherthe heater 14 is the non-electrified state (step 156).

If it is determined that the heater 14 is the non-electrified state, itcan be judged that the cause of the remoteness of the elementtemperature estimated value Tex from the element temperature theoreticalvalue Ti is not deterioration of the heater 14. In this case, the ECU 20identifies the content of deterioration of the oxygen sensor 10 as anabnormality of the sensor element 12 (step 158), and then ends thepresent execution cycle. The abnormality identification process of step158 is accomplished by a sensor element abnormality identifier that isprovided in the abnormality determiner.

Conversely, if it is determined in step 156 that the heater 14 is not inthe non-electrified state, the heater 14 is forcibly switched to thenon-electrified state (step 160). The diselectrification process of thestep 160 is accomplished by the electrification stopper that is providedin the abnormality determiner.

The forcible setting of the heater 14 to the non-electrified stateexcludes deterioration of the heater 14 from the potential cause of theremoteness of the element temperature estimated value Tex from theelement temperature theoretical value Ti. After step 160, the ECU 20calculates an element temperature estimated value Tex and an elementtemperature theoretical value Ti again, and determines whether thedifference |Tex−Ti| between the newly calculated values Tex, Ti isgreater than the criterion T3 (step 162).

If it is determined in step 162 that the |Tex−Ti|>T3 holds, it can bejudged that the cause of the remoteness between Tex and Ti is notabnormality of the heater 14. In this case, the ECU 20 executes theprocessing of step 158 to identify the content of deterioration of theoxygen sensor 10 as an abnormality of the sensor element 12, and thenends the present execution cycle.

Conversely, if it is determined in step 162 that |Tex−Ti|>T3 does nothold, it can be judged that the relationship between the elementtemperature estimated value Tex and the element temperature theoreticalvalue Ti has returned to a normal relationship due to the forceddiselectrification of the heater 14. In this case, the ECU 20 identifiesthe content of deterioration of the oxygen sensor 10 as an abnormalityof the heater 14 (step 164), and then ends the present execution cycle.This abnormality identification process is accomplished by a heaterelectrification state detector provided in the abnormality determiner.

FIG. 7 shows a flowchart illustrating an intermediate short circuitdetection process executed in step 146 by the ECU 20.

In the routine illustrated in FIG. 7, initial values of the elementtemperature estimated value Tex and the element impedance real value Rsrare stored (step 170).

Specifically, in step 170, the element temperature estimated value Texand the element impedance real value Rsr determined at a time point(point A) of the first execution of this routine are stored as a point-Aelement temperature estimated value TexA and a point-A element impedancereal value RsrA.

Subsequently, the present element temperature estimated value Tex isstored as a point-B element temperature estimated value TexB (step 172).

Subsequently, it is determined whether the difference TexA−TexB betweenthe point-A element temperature estimated value TexA and the point-Belement temperature estimated value TexB is greater than a predeterminedcriterion temperature T4 (e.g., 100° C.). That is, it is determinedwhether a change greater than the criterion temperature T4 has occurredon the element temperature estimated value Tex during the period fromthe detection of the point-A element temperature estimated value TexA tothe present execution cycle (step 174). A process of detecting theamount of change in the element temperature estimated value Tex in step174 is accomplished by the aforementioned second change amount detector.

The processing of step 174 is repeatedly executed until it is determinedthat TexA−TexB>T4 holds. When it is determined that this condition ismet, the element impedance real value Rsr at that time point ofdetermination is stored as a point-B element impedance real value RsrB(step 176).

In the routine illustrated in FIG. 7, it is subsequently determinedwhether the difference between the point-A element impedance real valueRsrA stored in step 170 and the point-B element impedance real value Rsrstored in step 176 is less than an intermediate short-circuit criterionR4 (step 178). A process of detecting the amount of change in theelement impedance real value Rsr in steps 178 is accomplished by theaforementioned first change amount detector.

The element temperature and the element impedance of the oxygen sensor10 should have a relationship corresponding to the normal temperaturecharacteristic. Therefore, as long as the oxygen sensor 10 is normal, achange in the element temperature estimated value Tex which is greaterthan the criterion temperature T4 should be accompanied by a change inthe element impedance real value Rsr which corresponds to the criteriontemperature T4.

The intermediate short-circuit criterion R4 used in step 178 is set at apredetermined value that is sufficiently small compared with the amountof change corresponding to the criterion temperature T4, specifically,an amount of change that should occur in the element impedance Rs whenan element temperature change of T4/2 (50° C.) occurs on the oxygensensor 10. Therefore, if it is determined that RsrA−RsfB<R4 holds, itcan be judged that the amount of change in the element impedance realvalue Rsr is excessively small with respect to the amount of change thathas occurred in the element temperature estimated value Tex.

In the routine illustrated in FIG. 7, if it is determined in step 178that RsrA−RsfB<R4 holds, it is judged that the oxygen sensor 10 has anintermediate short circuit (step 180).

Conversely, if it is determined that RsrA−RsfB<R4 does not hold, it isjudged that the oxygen sensor 10 does not have an intermediate shortcircuit. Hence, after it is determined that the oxygen sensor 10 isnormal, the present execution cycle ends (step 182).

As is apparent from the foregoing description, it is possible toprecisely determine whether there is a broken wire, a short circuit,deterioration, or an intermediate short circuit related to the oxygensensor 10, according to the routines illustrated in FIGS. 4 to 7. Thus,the apparatus of the embodiment is able to determine whether the oxygensensor 10 has an abnormality constantly with high precision, withoutbeing affected by the temperature characteristic of the elementimpedance real value Rsr, on the basis of the element impedance realvalue Rsr (or the element temperature theoretical value Ti) determinedfrom a value related to the electric power supplied to the oxygen sensor10 and the element temperature estimated value Tex calculated from avalue that affects the temperature of the oxygen sensor 10.

In the embodiment described above, the determination regarding a brokenwire and a short circuit and the determination regarding an intermediateshort circuit (see FIGS. 4 and 7) are based on comparison between theelement impedance real value Rsr and the element temperature estimatedvalue Tex, and the determination regarding deterioration is based oncomparison between the element temperature theoretical value Ti and theelement temperature estimated value Tex. However, this fashion ofdetermination is merely illustrative.

That is, in the embodiment, it is a precondition that the elementimpedance Rs of the oxygen sensor 10 has a temperature dependency.Therefore, since it is possible to calculate the element impedance Rs(the element impedance real value Rsr) from a value related to electricpower, it is also possible to calculate an element temperature (theelement temperature theoretical value Ti) from the electricpower-related value. Furthermore, since it is possible to calculate theelement temperature (the element temperature estimated value Tex) from avalue that affects temperature, it is also possible to calculate anelement impedance (hereinafter, referred to as “element impedanceestimated value”) on the basis of the value that affects temperature.

What is needed in order to determine whether there is a broken wire, ashort circuit, deterioration or an intermediate short circuit related tothe oxygen sensor 10 is that it can be determined whether there is anormal correlation between the element impedance Rs or the elementtemperature calculated from an electric power-related value and theelement impedance Rs or the element temperature calculated from a valuethat affects temperature. Therefore, the aforementioned determinationcan be performed on the basis of any one of the following fourcombinations:

-   (a) the combination of the element impedance real value Rsr and the    element temperature estimated value Tex;-   (b) the combination of the element impedance real value Rsr and the    element impedance estimated value;-   (c) the combination of the element temperature theoretical value Ti    and the element impedance estimated value; and-   (d) the combination of the element temperature theoretical value Ti    and the element temperature estimated value Tex.

The combination (a) corresponds to first deterioration detectionapparatus and method for an oxygen sensor of the invention. Thecombination (b) corresponds to second deterioration detection apparatusand method for an oxygen sensor of the invention. The combination (c)corresponds to third deterioration detection apparatus and method for anoxygen sensor of the invention. The combination (d) corresponds tofourth deterioration detection apparatus and method for an oxygensensor.

In the embodiment, the process of calculating an element temperaturetheoretical value Ti and acquiring the calculated value as a criterionis accomplished by the ECU 20 executing the process of step 150.Furthermore, the process of calculating an element impedance estimatedvalue and acquiring the calculated value as a criterion is accomplishedby the ECU 20 executing the process of step 100.

With the above-described construction, the invention achieves advantagesas follows. According to the first oxygen sensor deterioration detectionapparatus and method of the invention, whether the oxygen sensor has anabnormality can be determined constantly with high precision, withoutbeing affected by the temperature characteristic of the elementimpedance, on the basis of whether the element impedance real valuecalculated from a value related to the electric power supplied to theoxygen sensor and the element temperature estimated value calculatedfrom a factor that affects the temperature of the oxygen sensor have aproper relationship.

According to the second oxygen sensor deterioration detection apparatusand method, whether the oxygen sensor has an abnormality can bedetermined constantly with high precision, without being affected by thetemperature characteristic of the element impedance, on the basis ofwhether the element impedance real value calculated from a value relatedto the electric power supplied to the oxygen sensor and the elementimpedance estimated value calculated from a factor that affects thetemperature of the oxygen sensor have a proper relationship.

According to the third oxygen sensor deterioration detection apparatusand method, whether the oxygen sensor has an abnormality can bedetermined constantly with high precision, without being affected by thetemperature characteristic of the element impedance, on the basis ofwhether the element temperature theoretical value calculated from avalue related to the electric power supplied to the oxygen sensor andthe element impedance estimated value calculated from a factor thataffects the temperature of the oxygen sensor have a proper relationship.

According to the fourth oxygen sensor deterioration detection apparatusand method, whether the oxygen sensor has an abnormality can bedetermined constantly with high precision, without being affected by thetemperature characteristic of the element impedance, on the basis ofwhether the element temperature theoretical value calculated from avalue related to the electric power supplied to the oxygen sensor andthe element temperature estimated value calculated from a factor thataffects the temperature of the oxygen sensor have a proper relationship.

Furthermore, according to deterioration detection apparatus and methodin accordance with the invention, whether an oxygen sensor has anabnormality can be determined with high precision on the basis ofwhether the relationship between the element impedance real value andthe element temperature estimated value accords with a propertemperature characteristic, or on the basis of whether the relationshipbetween the element temperature theoretical value and the elementimpedance estimated value accords with a proper temperaturecharacteristic.

Still further, according to deterioration detection apparatus and methodin accordance with the invention, whether an oxygen sensor has anabnormality can be determined with high precision on the basis ofwhether the element impedance real value and the element impedanceestimated value are substantially equal, or on the basis of whether theelement temperature theoretical value and the element temperatureestimated value are substantially equal.

Further, according to deterioration detection apparatus and method ofthe invention, if it is determined that the oxygen sensor has anabnormality in a situation where the heater is not electrified, theabnormality can be identified as an abnormality of the sensor element.

Further, according to deterioration detection apparatus and method ofthe invention, if it is determined that the oxygen sensor has anabnormality in a situation where the heater is electrified, the state ofthe heater can be excluded from the cause of the abnormality by stoppingthe electrification of the heater.

Further, according to deterioration detection apparatus and method ofthe invention, if the determination of an abnormality of the oxygensensor is overturned by the exclusion of the state of the heater fromthe cause of the abnormality, it can be determined that the heater hasan abnormality.

Further, according to deterioration detection apparatus and method ofthe invention, if the first judgment value and the second judgment valuethat should change in a normal correlation therebetween are not found tohave the normal correlation, it can be determined that the oxygen sensorhas an abnormality. The above-described techniques of determination makeit possible to determine whether the oxygen sensor has an abnormalityconstantly with high precision without being affected by the temperaturecharacteristic of the element impedance.

While the invention has been described with reference to exemplaryembodiments thereof, it is to be understood that the invention is notlimited to the exemplary embodiments or constructions. To the contrary,the invention is intended to cover various modifications and equivalentarrangements. In addition, while the various elements of the exemplaryembodiments are shown in various combinations and configurations, whichare exemplary, other combinations and configurations, including more,less or only a single element, are also within the spirit and scope ofthe invention.

1. A degradation detection method for an oxygen sensor, comprising thesteps of: calculating an element impedance real value from a valuerelated to an electric power supplied to the oxygen sensor, andacquiring the calculated value as a first judgment value; calculating anelement temperature estimated value of the oxygen sensor from a factorthat affects a temperature of the oxygen sensor, and acquiring thecalculated value as a second judgment value; and determining whether theoxygen sensor has an abnormality based on the first judgment value andthe second judgment value, wherein, in the abnormality determining step,it is determined that the oxygen sensor has an abnormality if arelationship between the first judgment value and the second judgmentvalue does not agree with a normal temperature characteristic that isexhibited by an element impedance of the oxygen sensor, wherein theoxygen sensor comprises a sensor element that exhibits the temperaturecharacteristic, and a heater for heating the sensor element, wherein thefactor that affects the temperature of the oxygen sensor includes atleast a factor related to a state of operation of the heater, andwherein the abnormality determining step comprises the step of detectinga state of electrification of the heater, and the step of, if it isdetermined that the oxygen sensor has an abnormality in a situationwhere the heater is not electrified, identifying the abnormality as anabnormality of the sensor element.
 2. The degradation detection methodaccording to claim 1, wherein the abnormality determining step comprisesthe step of stopping electrification of the heater if it is determinedthat the oxygen sensor has an abnormality in a situation where theheater is electrified.
 3. The degradation detection method according toclaim 2, wherein the abnormality determining step comprises the step ofdetermining that the heater has an abnormality if determination of anabnormality of the oxygen sensor is overturned after the electrificationof the heater is stopped in the stopping step.
 4. The degradationdetection method according to claim 1, wherein the abnormalitydetermining step comprises the step of detecting an amount of change inthe first judgment value, and the step of detecting an amount of changein the second judgment value, and wherein it is determined that theoxygen sensor has an abnormality if the amount of change in the firstjudgment value and the amount of change in the second judgment value donot exhibit a normal correlation.
 5. A degradation detection method foran oxygen sensor, comprising the steps of: calculating an elementimpedance real value from a value related to an electric power suppliedto the oxygen sensor, and acquiring the calculated value as a firstjudgment value; calculating an element impedance estimated value from afactor that affects a temperature of the oxygen sensor, and acquiringthe calculated value as a second judgment value; and determining whetherthe oxygen sensor has an abnormality based on the first judgment valueand the second judgment value, wherein in the abnormality determiningstep, it is determined that the oxygen sensor has an abnormality if adifference between the first judgment value and the second judgmentvalue exceeds a predetermined criterion, wherein the oxygen sensorcomprises a sensor element that exhibits a temperature characteristic,and a heater for heating the sensor element, wherein the factor thataffects the temperature of the oxygen sensor includes at least a factorrelated to a state of operation of the heater, and wherein theabnormality determining step comprises the step of detecting a state ofelectrification of the heater, and the step of, if it is determined thatthe oxygen sensor has an abnormality in a situation where the heater isnot electrified, identifying the abnormality as an abnormality of thesensor element.
 6. The degradation detection method according to claim5, wherein the abnormality determining step comprises the step ofstopping electrification of the heater if it is determined that theoxygen sensor has an abnormality in a situation where the heater iselectrified.
 7. The degradation detection method according to claim 6,wherein the abnormality determining step comprises the step ofdetermining that the heater has an abnormality if determination of anabnormality of the oxygen sensor is overturned after the electrificationof the heater is stopped in the stopping step.
 8. The degradationdetection method according to claim 5, wherein the abnormalitydetermining step comprises the step of detecting an amount of change inthe first judgment value, and the step of detecting an amount of changein the second judgment value, and wherein it is determined that theoxygen sensor has an abnormality if the amount of change in the firstjudgment value and the amount of change in the second judgment value donot exhibit a normal correlation.
 9. A degradation detection method foran oxygen sensor, comprising the steps of: calculating an elementtemperature theoretical value from a value related to an electric powersupplied to the oxygen sensor, and acquiring the calculated value as afirst judgment value; calculating an element impedance estimated valuefrom a factor that affects a temperature of the oxygen sensor, andacquiring the calculated value as a second judgment value; anddetermining whether the oxygen sensor has an abnormality based on thefirst judgment value and the second judgment value, wherein in theabnormality determining step, it is determined that the oxygen sensorhas an abnormality if a relationship between the first judgment valueand the second judgment value does not agree with a normal temperaturecharacteristic that is exhibited by an element impedance of the oxygensensor, wherein the oxygen sensor comprises a sensor element thatexhibits the temperature characteristic, and a heater for heating thesensor element, wherein the factor that affects the temperature of theoxygen sensor includes at least a factor related to a state of operationof the heater, and wherein the abnormality determining step comprisesthe step of detecting a state of electrification of the heater, and thestep of, if it is determined that the oxygen sensor has an abnormalityin a situation where the heater is not electrified, identifying theabnormality as an abnormality of the sensor element.
 10. The degradationdetection method according to claim 9, wherein the abnormalitydetermining step comprises the step of stopping electrification of theheater if it is determined that the oxygen sensor has an abnormality ina situation where the heater is electrified.
 11. The degradationdetection method according to claim 10, wherein the abnormalitydetermining step comprises the step of determining that the heater hasan abnormality if determination of an abnormality of the oxygen sensoris overturned after the electrification of the heater is stopped in thestopping step.
 12. The degradation detection method according to claim9, wherein the abnormality determining step comprises the step ofdetecting an amount of change in the first judgment value, and the stepof detecting an amount of change in the second judgment value, andwherein it is determined that the oxygen sensor has an abnormality ifthe amount of change in the first judgment value and the amount ofchange in the second judgment value do not exhibit a normal correlation.13. A degradation detection method for an oxygen sensor, comprising thesteps of: calculating an element temperature theoretical value from avalue related to an electric power supplied to the oxygen sensor, andacquiring the calculated value as a first judgment value; calculating anelement temperature estimated value from a factor that affects atemperature of the oxygen sensor, and acquiring the calculated value asa second judgment value; and determining whether the oxygen sensor hasan abnormality based on the first judgment value and the second judgmentvalue, wherein in the abnormality determining step, it is determinedthat the oxygen sensor has an abnormality if a difference between thefirst judgment value and the second judgment value exceeds apredetermined criterion, wherein the oxygen sensor comprises a sensorelement that exhibits a temperature characteristic, and a heater forheating the sensor element, wherein the factor that affects thetemperature of the oxygen sensor includes at least a factor related to astate of operation of the heater, and wherein the abnormalitydetermining step comprises the step of detecting a state ofelectrification of the heater, and the step of, if it is determined thatthe oxygen sensor has an abnormality in a situation where the heater isnot electrified, identifying the abnormality as an abnormality of thesensor element.
 14. The degradation detection method according to claim13, wherein the abnormality determining step comprises the step ofstopping electrification of the heater if it is determined that theoxygen sensor has an abnormality in a situation where the heater iselectrified.
 15. The degradation detection method according to claim 14,wherein the abnormality determining step comprises the step ofdetermining that the heater has an abnormality if determination of anabnormality of the oxygen sensor is overturned after the electrificationof the heater is stopped in the stopping step.
 16. The degradationdetection method according to claim 13, wherein the abnormalitydetermining step comprises the step of detecting an amount of change inthe first judgment value, and the step of detecting an amount of changein the second judgment value, and wherein it is determined that theoxygen sensor has an abnormality if the amount of change in the firstjudgment value and the amount of change in the second judgment value donot exhibit a normal correlation.